The D. O. Melroy et al. U.S. Pat. No. 3,607,679 discloses a relatively complex method of fabricating an RC network in which a layer of beta tantalum is deposited on a substrate and then selectively etched in a masking process to define a resistor window and a capacitor slit. An initial layer of a tantalum pentoxide capacitor dielectric next is formed on a beta tantalum capacitor electrode by masking and anodization to provide an etch stop for the electrode during subsequent processing. Layers of tantalum nitride and a contact film base conductor (e.g., tantalum-gold or nickel-chromium-gold) then are sequentially sputtered on the resultant assembly. The contact film base conductor then is masked and selectively etched to define resistor and capacitor areas. Next, the assembly is masked and tantalum nitride is etched from the resistor area to form a resistor pattern, and from the capacitor dielectric area to permit final forming of the capacitor dielectric in a subsequent anodization process. During the etching of the tantalum nitride, the initial capacitor dielectric functions as an etch stop to protect the beta tantalum capacitor electrode as noted previously.
The assembly next is masked and a portion of the tantalum nitride film is anodized to form a resistor, after which the assembly is heat treated to thermally stabilize the resistor. Following the heat treatment, the assembly is again masked and the capacitor dielectric area of the circuit is reanodized to complete formation of the capacitor dielectric. An electrically conductive film (e.g., nickel-chromium-gold) then is deposited on the resultant assembly. Finally, the electrically conductive film is selectively etched in a masking process to define a conductor network and a capacitor counterelectrode.
In another embodiment in the Melroy et al. patent, the capacitor and the resistor are formed in a reverse order from the embodiment described hereinabove. Initially, a resistor is formed by anodizing a portion of a tantalum nitride film which has been deposited on a substrate, after which the resultant assembly is heat treated to thermally stabilize the resistor. A beta tantalum film then is deposited on the assembly, and the beta tantalum film and the tantalum nitride film are selectively etched to define a capacitor slit in the films. A portion of the beta tantalum film then is anodized to produce a tantalum pentoxide capacitor dielectric. Next, an electrically conductive film (e.g., nickel-chromium-gold) is plated on the assembly and then selectively etched to define a capacitor counterelectrode and a conductor pattern.
The Anders U.S. Pat. No. 4,058,445 discloses a method of producing a thin film tantalum capacitor utilizing an alpha tantalum base electrode. A tantalum pentoxide capacitor dielectric then is grown on the base electrode by oxidation. The dielectric then is covered with another electrode, whereupon the completed capacitor is heat treated. The Anders patent also suggests that with the method disclosed therein it is possible to fabricate RC networks, with the resistor and capacitor films being deposited directly one after another.
A known process for fabricating a thin film RC network utilizing alpha tantalum is patterned after the above-described first embodiment of the Melroy et al. patent. For example, in the fabricating of a plurality of RC networks on a ceramic substrate, this process involves the initial steps of selectively glazing the substrate in proposed capacitor areas, sputtering a tantalum film on the substrate, thermally oxidizing the tantalum film to form a deposit underlay, and then sputtering an alpha tantalum film over the deposit underlay. The alpha tantalum film next is masked and selectively etched to define a capacitor base electrode for each of the networks and to define anodization bus bars which are connected to the capacitor base electrodes. A tantalum pentoxide capacitor dielectric then is grown on the capacitor base electrode by masking and anodizing to form an etch stop during subsequent processing.
A tantalum nitride resistor film next is sputtered on the resultant assembly, and films of titanium and palladium are sequentially sputtered over the resistor film to insure good ohmic contact during subsequent processing. The titanium and palladium films then are selectively etched in a masking process to define contact spots for the resistors and capacitors subsequently to be formed. The tantalum nitride film next is selectively etched in a masking process to define the resistors, and to expose the initially formed capacitor dielectric etch stop layer, after which the formation of the capacitor dielectric is completed in a second masking and anodizing step.
After heat treatment to thermally stabilize the resistance and capacitance properties of the RC networks, any palladium oxide which has formed on the contact spots for the resistors and capacitors is reduced to restore good ohmic contact for the resistors and capacitors. Nickel-chromium and palladium films then are sequentially sputtered on the assembly to produce a dual adhesive "glue" layer which facilitates adhesion of gold to the assembly, after which a gold film is plated on the adhesive layer. Next, the electrically conductive films are masked and selectively etched to define conductor networks and capacitor counterelectrodes. The alpha tantalum anodization bus bars then are removed by masking and etching in a separate etching operation. Subsequently, the substrate is scribed to separate the completed RC networks into discrete independent units.
The above-described method of forming thin film RC networks utilizing alpha tantalum has a number of disadvantages. For example, the method involves a large number of processing steps, including a large number of mask levels, resulting in a relatively complex process. For example, the formation of the capacitance dielectrics is accomplished in two separate masking and anodization steps, the first to provide an etch stop for the capacitor base electrodes and the second to complete the formation of the dielectrics. Further, since the formation of the tantalum nitride film and the electrically conductive film are separated by intervening steps, the formation of titanium-palladium contact spots, which involves sputtering, masking and etching steps, and the reduction of palladium oxide on the contact spots after heat treatment of the networks, is required to insure good ohmic contact between the tantalum nitride film and the subsequently formed resistors and capacitors. In addition, removal of the alpha tantalum anodization bus bars must be accomplished in a separate masking and etching operation. As a net result, the described method involves a significant amount of labor and material expense, with a long manufacturing interval.
Accordingly, a purpose of this invention is to provide a relatively simple and economical method of fabricating a thin film RC network of high precision and reliability with a reduction in the number of processing steps and mask levels involved.